Opposition parties have called for an emergency meeting of the House of Commons defence committee on Tuesday so they can be briefed on how Canada is responding to the threat posed by North Korea.

The request comes after North Korea tested a second intercontinental ballistic missile this month, sparking warnings and ultimatums between Pyongyang and U.S. President Donald Trump.

Yet much of the discussion is expected to be on whether the Liberal government intends to reverse Canada's previous decision and join the U.S. military's controversial ballistic missile defence system.

"What I want to hear from the government is what are their plans (for BMD), and will they stand by the Canadian government's long-standing policy," said NDP foreign affairs critic Helene Laverdiere.

"That is what is unclear and has to be discussed by parliamentarians."

The U.S. invited Canada more than a decade ago to participate in its missile-defence system, but then-prime minister Paul Martin opted out following an extremely heated national debate in 2005.

The issue remained largely off the radar for more than a decade until the Trudeau Liberals asked defence experts and others to weigh in last year on whether Canada should reverse its earlier decision.

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Former prime minister Paul Martin opted out of including Canada in the U.S.'s missile-defence system after an extremely heated national debate in 2005.

The question was one of several raised during public consultations for the Liberals' new defence policy.

The Canadian military, numerous defence experts as well as the Commons' and Senate defence committees in separate reports have supported Canada's participation.

Yet when it was released in June, the defence policy made virtually no mention of missile defence.

The Trudeau government has since sidestepped repeated questions about the Liberals' intentions when it comes to BMD —- including whether Canadian officials have talked to their U.S. counterparts about it.

Defence Minister Harjit Sajjan's office instead says ballistic missiles are only one threat being discussed as Canada and the U.S. look to modernize North America's aging early-warning air defence system.

"The government of Canada has already committed to examining, through NORAD modernization, territorial defence against all perils," Sajjan spokeswoman Jordan Owens said in an email.

Feds have been 'very vague': expert

That includes, Owens said, "threats from cruise missiles, ballistic missiles and other future technologies to provide Canadians with greater security at home."

The Liberal ambiguity has surprised observers such as James Fergusson, director of the University of Manitoba's Centre for Defence and Security Studies and a leading authority on missile defence.

"I expected that the government would say something more, or at least be a little more direct about engaging the United States in discussions on missile defence," Fergusson said.

"Instead, they've been very, very vague and ambiguous."

It has also ruffled feathers on both sides of the debate, with both the NDP, which opposes Canada's participation, and the Conservatives, who support it, now demanding clarity.

"Canadians expect Canada to be part of BMD," said Conservative defence critic James Bezan.

System has had mixed success in tests

"Now it's up to the government on how they approach that sensitive subject with the United States. If they're waiting for an invitation to join, I don't think it's going to happen."

Now might actually be the ideal time to broach the subject with the U.S., as Defense Secretary James Mattis is conducting a year-long review on how to improve the $100-billion missile-defence shield.

Yet any move in that direction is likely to spark an uproar from those who oppose continental missile defence and see it as a trigger for a new nuclear arms race.

The U.S. system is comprised of land- and sea-based interceptors that would stop a limited missile attack, such as what North Korea could launch, but would be useless against an all-out assault by China or Russia.

Despite the amount of money involved, the system has had only mixed success over the years intercepting ballistic missiles in tests.